Chemical Senses Flashcards
List the chemical senses in the body
- Taste
- Smell
- Chemical irritants
- CO2/O2 levels
- Acidity
How can our taste be innate? (In born)
- Some of our taste preferences are inborn
- Humans innately enjoy sweet flavours and avoid bitter flavours, this is evolutionary ancient
How can our taste be learned?
- Our experience can also modify our innate preferences
- Humans can learn to tolerate or enjoy the bitterness of some substances
How can we perceive flavour?
Via 3 different systems
- Smell
- Touch: Texture, temperature
- Taste: Sweet, Bitter, Sour, Salt, Umami
Define taste
- Primarily a function to the tongue
- However other areas of the mouth such as the mouth, throat and nasal passages also have important roles in taste
What are the organs of taste?
- Palate: Roof of mouth separating oral and nasal cavities, taste buds present in palate
- Epiglottis: Leaf shaped cartilage covering laryngeal inlet, taste buds present here
- Pharynx and Nasal cavity: Odoura can pass via the pharynx, to the nasal cavity to be detected by olfactory receptors
Describe the features of the tongue
The surface of the tongue contain papillae
- Ridge shaped (foliate)
- Pimple shaped (vollate)
- Mushroom shaped (fungiform)
What does the papillae also contain?
- Contains taste receptors cells
- These taste buds are surrounded by basal cells and gustatory afferent axons
Describe taste receptor cells
- Express different types of taste receptors
- Most taste receptor cells respond primarily to one of the 5 basic tastes
How does taste receptor cells work?
- Three taste receptor cells sequentially exposed to Salt, bitter, sour and sweet stimuli
- Membrane potential recorded
- Taste receptors cells display different sensitivities
- Taste receptor cells form synapses with gustatory afferent axons to transmit this gustatory information
Name the 2 mechanisms for taste transduction
- Saltines
- Sourness
Briefly describe saltiness
The prototypical salty chemical is table salt (NaCl)
- Taste of salt is mostly the taste of the cation sodium (Na+)
How does Saltiness act as a transduction mechanism?
- Na+ passes through Na+ selective channels down its concentration gradient
- This depolarises the taste cell, activating voltage gated calcium channels
- Vesicular release of neurotransmitters is elicited and gustatory afferent activated
Why are special Na+ selective channels used?
- Used to detect low concentrations of salt
- Insensitive to voltage and generally stays open
Briefly describe sourness
Protons (H+) are the determinants of acidity and sourness
How does sourness act as a transduction mechanism?
- H+ can affect sensitive taste receptors in several ways
- However it’s likely that H+ can pass through proton channels and bind to and block K+ selective channels
- This leads to depolarisation of the taste cell activating VGSC & VGCCs
- Vesicular release of neurotransmitters is elicited and gustatory afferent activated
Name the different mechanisms of taste transduction (GPCR mechanisms via T1 and T2 taste receptors)
Bitterness
Sweetness
Umami
Briefly describe taste receptor proteins (T1R and T2R)
Transduction mechanisms underlying bitter, sweet and umami tastes rely on two families of related taste receptor proteins: T1Rs and T2Rs
Describe how taste receptor proteins work
T1R and T2Rs are G protein coupled receptors and are Gq linked
1. Bitter substances are detected by approximately 25 T2Rs
2. Sweet substances are detected by one receptor (T1R2 & T1R3 proteins)
3. Umami substances are detected by one receptor (T1R1 and T1R3 proteins)
What is the bitterness transduction mechanism? (PART 1)
- Bitter tastings binds to T2R which is a G protein Gq
- This stimulates the enzyme phospholipids C (PLC) leading to the production of inositol triphosphate (IP3)
What is the bitterness transduction mechanism? (PART 2)
- IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from intracellular storage sites
- Both these actions depolarise the taste cell – release of ATP is elicited, and gustatory afferents are activated
What is the sweetness transduction mechanism?
- Sweet tastants binds to diner receptor formed T1R2 and T1R3 which is coupled by G protein Gq linked
- The same signal transduction mechanism as bitterness occurs
Why do we not get confused between bitter and sweet tastes?
- Taste cells express either bitter or sweet receptors
- Not both
- This causes the bitter and sweet taste cells to connect to different gustatory axons
What is the Umami taste transduction mechanism?
- Umami tastants bind to diner receptor formed from T1R1 and T1R3 which is coupled to the G protein Gq
- The same signal transduction mechanism as bitterness and sweetness occurs
- Shares T1R3 protein with sweetness
- T1R subunit determines specificity to umami
Why do we not get confused between bitter, sweet and umami tastes?
- Taste cells express either bitter, sweet or umami receptors
- Therefore bitter, sweet and umami taste cells connect to different gustatory axons
What is the flow of taste information to the CNS?
- Taste cells -> Gustatory axons -> Gustatory nucleus (Medulla) -> Ventral posterior medial nucleus (Thalamus) -> Gustatory cortex
Which three cranial nerves carry gustatory axons and bring taste information to the brain?
Facial Nerve
Glossopharyngeal Nerve
Vagus Nerve
How can our smell be innate?
Some of our smell preferences can be inborn
How can smell be learned?
Experience can strongly modify our innate preferences
What are pheromones?
An Olfactory stimuli used for chemical communication between individuals
Give some examples of where pheromones are used
In some animals, Phermomes are important signals for:
- Reproductive behaviour
- Marking territories
- Indicating Aggression/Submission
Where can Olfactory receptor cells be found?
At the site of transduction
What is the function of Olfactory receptor cells?
- Function is to produce mucus
- Odorants dissolve in mucus layer before contacting cilia of olfactory receptor cells
What does the olfactory receptor cells do in the Basel cells?
- Immature olfactory receptor cells are able to differentiate into mature olfactory receptor cells
- Olfactory receptor cells continuously grow, degenerate and regenerate
What is the Olfactory transduction mechanism?
- Odorant molecules bind to odorant receptor proteins on the cilia
- Olfactory-specific G-protein (Golf) is activated
- Adenylyl cyclase activation increases cAMP formation
- cAMP-activated channels open, allowing Na+ and Ca2+ influx
- Ca2+ activated chloride channels open enabling Cl- efflux
- Causes membrane depolarisation of the olfactory neuron
What is the flow of smell information to the CNS?
- Olfactory receptor cells send axons into the olfactory bulb
- Olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb
- Signals are relayed in the glomeruli and transmitted to higher regions of the brain
What happens in population coding?
The response of a large number of broadly tuned neurons are used to specify the properties of particular stimuli
How can the brain distinguish between specific tastes and smells in Gustation and Olfaction stimuli? (Population coding)
- Gastatory and olfactory receptor cells may express only one specific receptor protein
- However Gustatory and Olfactory axons along with the neurons they activate in the brain, respond more broadly
- Only with large population of neurons, with different response patterns, can the brain distinguish between specific tastes and smells
Give an example of population coding for olfaction (PART 1)
- Olfactory receptor cells express a single olfactory receptor protein – each can respond to different odours with differing preferences
- When presented with a citrus smell, none of the three receptor cells can individually distinguish it from the other odours
Give an example of population coding for olfaction (PART 2)
- However, the brain can distinguish the citrus smell through the combination of responses from all three cells
- It is estimated that humans can discriminate at least one trillion different combinations of odour stimuli
